A parallel world is another universe situated "alongside" our own, displaced from it along a spatial fourth Dimension (parallel worlds are often referred to in sf as "other dimensions"). Although whole universes may lie parallel in this sense, most stories focus on parallel Earths. The parallel-world idea forms a useful framework for the notion of Alternate History, and is often used in this way. Most of the "secondary worlds" of modern Fantasy are explicit or implicit parallel worlds. Notable early sf extrapolations include J-H Rosny aîné's Un autre monde (1895 Revue Parisienne #5; exp as coll 1898; trans as "Another World" in A Century of Science Fiction, anth 1962, ed Damon Knight) – whose "other world" is not truly separate from Earth but merely hidden from normal human perception – and two stories by H G Wells: "The Remarkable Case of Davidson's Eyes" (28 March 1895 Pall Mall Budget) and "The Plattner Story" (April 1896 The New Review).
The idea that other worlds lie parallel to our own and occasionally connect with it is one of the oldest speculative ideas in literature and legend; examples range from Fairyland to the "astral plane" of Spiritualists and mystics. There are two basic folkloristic themes connected with the notion; in one, an ordinary human is translocated into a fantasy land where s/he undergoes adventures and may find the love and fulfilment that remain beyond reach on Earth; in the other, a communication or visitation from the other world affects the life of an individual within this world, often injuring or destroying that person. Both patterns are very evident in modern imaginative fiction, shaping whole subgenres. Much of the overlap between sf, Fantasy and Horror fiction – which makes clear-cut Definitions of the genres impossible – occurs by virtue of the promiscuous use of parallel worlds. The first pattern was modernized by Edgar Rice Burroughs, A Merritt and other Pulp-magazine writers before the founding of Amazing Stories, and was easily dressed up with pseudoscientific jargon; a notable early example is The Blind Spot (14 May-18 June 1921 Argosy; 1951) by Homer Eon Flint and Austin Hall. Henry Kuttner and C L Moore wrote several Merrittesque Science-Fantasy novels after this fashion, notably The Dark World (Summer 1946 Startling; 1965) and Beyond Earth's Gates (September 1949 Startling as "The Portal in the Picture"; 1954 dos). Among the first writers to co-opt parallel worlds for straightforward sf melodrama were Edmond Hamilton, in "Locked Worlds" (Spring 1929 Amazing Stories Quarterly), and Murray Leinster, in "The Fifth-Dimension Catapult" (January 1931 Astounding) and its sequels. The idea was frequently used in humorous fashion by L Sprague de Camp and others in Unknown. The second pattern, in which entities from a parallel world impinge on ours, was science-fictionalized by William Hope Hodgson in The Ghost Pirates (1909); his earlier The House on the Borderland (1908) uses the landscapes of a parallel world to map and symbolically display the psyche of its protagonist. The renewal of such traditional horror motifs by sf imagery was taken further by H P Lovecraft in a manner imitated by his many disciples, including Frank Belknap Long and Donald Wandrei.
The early Genre-SF writers were slow to develop more extravagant speculative possibilities, although one notable attempt to describe a parallel world with different physical laws from those holding in our own continuum was made by Clark Ashton Smith in "The Dimension of Chance" (November 1932 Wonder Stories); this notion was eventually developed much more carefully and elaborately by Isaac Asimov in The Gods Themselves (March/April-May-June 1972 Galaxy; 1972). Raymond F Jones's Renaissance (July-October 1944 Astounding; 1951; vt Man of Two Worlds 1963) is straightforward, and Fritz Leiber's use of parallel alternative worlds in Destiny Times Three (March-April 1945 Astounding; 1957) is quantitatively restrained. It was in the 1950s and 1960s that exploration of the quirkier corollaries of the basic notion really got under way. Clifford D Simak imagined a more extensive series of Earths – all empty of humanity and thus available for colonization and exploitation – in Ring Around the Sun (December 1952-February 1953 Galaxy; 1953); a similar scenario is explored in much more detail in Stephen Baxter's and Terry Pratchett's The Long Earth (2012). Simak examined the hazards of trading between parallel worlds in "Dusty Zebra" (September 1954 Galaxy) and "The Big Front Yard" (October 1958 Astounding), as did Alan E Nourse in "Tiger by the Tail" (November 1951 Galaxy). Gordon R Dickson's Delusion World (July 1955 Science Fiction Stories as "Perfectly Adjusted"; exp 1961) features a city simultaneously occupied by two societies, each invisible to the other.
A common variant of the theme is that of a multiplicity of almost-identical worlds existing in parallel: alternate worlds in which there has been no significant change. Examples include "The Celestial Plot" (in La trama celeste, coll 1948; trans in The Invention of Morel, coll 1964) by Adolfo Bioy Casares and "Next Door, Next World" (April 1961 Analog) by Robert Donald Locke. In Robert Silverberg's "Trips" (in Final Stage, anth 1974, ed Edward L Ferman & Barry N Malzberg) transuniversal tourists wander aimlessly through worlds similar and dissimilar. Parallel worlds often feature eccentric societies, sometimes for purposes of Satire, and sometimes equally eccentric patterns of Evolution – like that in Stephen Boyett's The Architect of Sleep (1986), where raccoons have become the dominant technological species. Bob Shaw has used the notion cleverly in two original novels: The Two-Timers (1968), in which a man who has lost his wife inadvertently creates a parallel world in which she still exists, and A Wreath of Stars (1976), in which two worlds made of different species of matter co-exist until the approach of an anti-neutrino star shifts the orbit of one of them. Another kind of parallelism is featured in a group of stories in which Timeslips bring different eras of earthly history into geographical proximity – a motif featured in "Sidewise in Time" (June 1934 Astounding) by Leinster and October the First is Too Late (1966) by Fred Hoyle. The idea that parallel worlds might include literal versions of fictional worlds as well as alternative histories is proposed in "The Number of the Beast" (1980) by Robert A Heinlein and more sensitively developed in Frankenstein Unbound (1973) by Brian W Aldiss. Larry Niven's "All the Myriad Ways" (October 1968 Galaxy) deals tentatively with the psychological implications of multiple universes. Richard Cowper's Breakthrough (1967) extrapolates the psychological attractions of the concept, as do Christopher Priest's stories of the Dream Archipelago, including The Affirmation (1981).
Modern uses of the theme usually imagine an infinite number of parallel worlds extending in a manifold which contains all possible Earthly histories and perhaps all possible physical universes (for more extended discussion see Alternate Cosmos). The notion that the perceived Universe is simply one single aspect of such a Multiverse has been lent credence by the "many-worlds interpretation" of the enigmas of quantum mechanics propounded by, for example, John Wheeler, and popularized in nonfiction books by such writers as Paul Davies and John Gribbin. Keith Laumer's Worlds of the Imperium (1962) and its sequels deploy this kind of infinite series of parallel worlds in connection with alternative histories, as do Richard C Meredith's At the Narrow Passage (1973) and its sequel and Frederik Pohl's The Coming of the Quantum Cats (1986). As the twenty-first century has progressed, there has been an increasing tendency to re-create more tolerable planetary histories through Alternate History devices, and whole subgenres like Steampunk; without any historical arguments, such retro impulses can lead to the creation of parallel worlds, such as the Planetary Romance version of Mars promulgated in Old Mars (anth 2013) edited by Gardner Dozois and George R R Martin.
Certain philosophical implications of the many-worlds interpretation are explored more-or-less seriously in a number of sf works, including Brian Aldiss's Report on Probability A (1968), Larry Niven's "All the Myriad Ways" (October 1968 Galaxy), Graham Dunstan Martin's Time-Slip (1986), Greg Egan's Quarantine (1992), Frederik Pohl's and Jack Williamson's The Singers of Time (1991), John Barnes's Finity (1999) and Stephen Baxter's Origin: Manifold 3 (2001; vt Manifold: Origin 2001). Neal Stephenson's Anathem (2008) ambitiously deals with the many-worlds view and the possibility of travel between alternatives from an extensive basis of Platonic and other philosophy; Adam Roberts's Yellow Blue Tibia (2009) uses a shifting multiplicity of parallel worlds to rationalize the notorious elusiveness of UFOs.
Modern fantasy novels – including most of those in the intermediate Science Fantasy and Science and Sorcery categories (which see) – sometimes draw upon the legacy of sf recomplication in order to invigorate their use of parallel worlds. Notable examples include Roger Zelazny's Amber series and Michael Moorcock's many Sword-and-Sorcery series, which are all bound together (with some sf novels) within a hypothetical Multiverse. [BS/DRL]
see also: GURPS; Half-Life; Omega Point.
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